The invention relates to a method and apparatus for facilitating the installation of a well. In particular, the invention is directed to the installation of a ground water monitoring well wherein the well casing and screen, the filter material and the sealing and back-fill materials are assembled in a clean area and thereafter placed as a preform in the boring.
In a conventional ground water monitoring well installation, a hole is bored in the ground and a clean casing, with the lower portion slotted or perforated to form a screen, is lowered into the bore hole. Granular filler material is poured into the annular space between the inside of the boring and the outside of the well casing. Enough filler material is added to cover all of the slotted screen portion of the wall casing. A clay barrier of low permeability is installed above the filter material by dropping pelletized clay (typically bentonite) into the annular space. If the bore hole is dry, water is poured therein to hydrate the pelletized clay in order to form a cap or seal above the filter material. Clean back-fill material (for example, soil from the bore hole) is used to fill the annular space to the ground level. The well is then surged and pumped to develop the filter material and wash out any fine soil particles that have mixed with the filter material.
Conventional techniques for installing ground water monitoring wells produce a number of problems and uncertainties in the final configuration. The bore hole may slough or cave pushing soil against the screen to thereby cause clogging. The filter material added from the top of the bore hole may bridge across the annular space and create voids opposite the screen slots so that the filter material is not properly located and the adjacent soil may clog the screen slots. The bentonite pellets may also bridge over the annular space and thus may not form a capping layer atop and adjacent the filter material. When the pellets do not form a continuous layer atop the filter material, fluids from the surface can be drawn down the annular space and through the filter material into the slots in the well screen. Thus, when the clay seal does not form properly, the well no longer takes fluids only from the soil adjacent to the screen through the filter material, but also receives fluids from above including surface water. In wells used for monitoring water quality, the samples are worthless if contamination caused by infiltration of surface water is present in the samples. The problem is equally serious when the well is used to obtain soil gas samples where air can be pulled into the infiltration zone and thereby dilute the soil gas.